Carpenters, electricians, do-it-yourselfers and others are often faced with the problem of locating the position of the wall studs behind the wall board material forming the wall surface. They are interested in hanging pictures, drilling holes and so on. However after the walls are finished and painted the location of the hidden substructure (i.e. the studs) is not visually detectable. The same is true of finding the location of hidden wooden frames in furniture and boats from the outside surface of the structure.
U.S. Pat. No. 4,099,118 issued Jul. 4, 1978 discloses an electronic wall stud sensor which is suitable for detecting a wall stud behind a wall surface. It utilizes one or more capacitor plates, a fixed frequency oscillator, a dual one-shot multivibrator, a field effect transistor, and a complicated calibration procedure. Each individual circuit must be calibrated at the time of manufacture, which is a costly procedure for mass production.
U.S. Pat. No. 4,464,622 describes a wall stud sensor similar to U.S. Pat. No. 4,099,118 but with a plurality of capacitor elements and means for detecting the presence of alternating current in the wall. Finding the presence of alternating current in walls is often not practical or possible with modern wiring methods. U.S. Pat. No. 5,352,974 describes a stud sensor similar to U.S. Pat. No. 4,099,118 but with means for storing calibration data for thick or thin walls. However, in most cases, the user will not know if the wall is thick or thin. The circuit used is complex and uses special purpose hardware. The sensor also uses a plurality of capacitor plates. Both of these devices require factory calibration.
U.S. Pat. No. 5,485,092 describes a device for investigating surface and subsurface structures. It uses four-sided conductive elongated plates and rectangular sensor plates connected together in a special arrangement. The different surfaces are charged at different rates and a differential amplifier and peak detector are used to determine information about the subsurface. It requires a complicated charging scheme and an expensive voltmeter for readout, which requires an interpretation of the results which would be difficult for an inexperienced person.
Prior art sensors were required to be a relatively large size so as to make them sufficiently sensitive for their intended purpose. Prior circuits required a relatively large sensor, and to isolate the sensor from the user's hand, which contributed to the relatively large size of the sensors.
Thus, there is a need for a low cost subsurface object locator that is easy to use, works well in the environment for which it is designed, simply and reliably identifies the location of substructure components in an efficient manner, is easy to manufacture, requires no calibration or adjustments by the factory or operator, and can be made of a relatively small size.